1. Field of the Invention
The present invention relates to an apparatus for controlling the amount of slip of a lock-up clutch provided in a power transmitting system of a motor vehicle, an an apparatus for cutting fuel suuply to an engine of the vehicle.
2. Discussion of the Related Art
In a motor vehicle having a fluid-filled power transmitting device equipped with a lock-up clutch such as a torque convertor or fluid coupling incorporating such a lock-up clutch, it is proposed to control the lock-up clutch in a slip control mode or partially engaging mode during acceleration of the vehicle such that an actual amount of slip (slip speed) of the lock-up clutch, namely, a difference between the speeds of a pump impeller and a turbine impeller eventually coincides with a predetermined target slip speed, for the purpose of improving the fuel economy of the vehicle while minimizing the power loss due to slipping of the lock-up clutch. Some motor vehicles are equipped with a fuel-cut device adapted to cut a fuel supply to the engine during deceleration or coasting of the vehicle while the engine speed is higher than a predetermined fuel-cut speed. On such motor vehicles, it is also proposed to control the lock-up clutch in the slip control mode during the vehicle deceleration, for raising the engine speed so as to increase a period of time during which the fuel supply to the engine is cut by the fuel-cut device.
The slip control of the lock-up clutch during the vehicle deceleration is effected as long as the vehicle deceleration is detected by an idling position switch which is adapted to be turned on when a throttle valve of the engine is placed in the idling position. Namely, the idling position switch placed in the on position indicates that the vehicle is in deceleration or in a coasting run. The slip control is terminated when an operation to cut the fuel supply to the engine by the fuel-cut device is completed. As a result of this slip control of the lock-up clutch, the engine speed is raised to a level which is lower than the speed of the turbine impeller by the target slip speed. Therefore, a time period during which the engine speed is held above the predetermined fuel-cut speed is elongated. That is, the fuel-cut period is increased by the slip control operation of the lock-up clutch during the vehicle deceleration, whereby the fuel economy is accordingly improved. An example of such a slip control apparatus is disclosed in JP-A-1-220765, JP-A-2-118265, JP-A-5-149423 and JP-A-5-141528.
In the slip control of the lock-up clutch during the vehicle deceleration or coasting, the amount of a torque transmitted through the lock-up clutch (hereinafter referred to as "lock-up clutch torque") is relatively small, and a small change in the friction force or engagement force of the lock-up clutch will cause a comparatively large amount of change of the slip speed of the lock-up clutch. Thus, the slip control of the lock-up clutch tends to be unstable particularly in an initial period of the slip control. Accordingly, the conventional slip control apparatus suffers from a relatively low degree of control stability due to a change in the output torque of the engine upon operation of the fuel-cut device to cut the fuel supply to the engine.
In the conventional slip control apparatus disclosed in JP-A-2-118265, for example, the amount of slip of the lock-up clutch is controlled when the vehicle deceleration is detected. The slip control operation is initiated only after the lock-up clutch is once brought to the fully released position, and then the fuel-cut device is activated to cut the fuel supply when a predetermined time has passed after the initiation of the slip control operation. Thus, when the vehicle deceleration is detected, the engine speed suddenly drops due to the full releasing of the lock-up clutch. This may lead to inadequate operation of the lock-up clutch in the slip control mode. In the conventional slip control apparatus disclosed in JP-A-1-220765, the slip control of the lock-up clutch is effected according to a control equation which includes a feedback control value and a feed-forward control value. The fuel-cut device may be held off for some reason or other, even after the slip control operation is initiated. For example, the fuel-cut device is held off while the temperature of engine coolant water is held low. In this case, when the coolant water temperature rises above a certain limit, the fuel-cut device is activated, with a result of a variation in the output torque of the engine. This variation in the engine torque due to the operation of the fuel-cut device may impair the slip control stability of the lock-up clutch.
The slip control of the lock-up clutch when effected together with the fuel-cut operation during vehicle deceleration with the engine in the idling state as disclosed in JP-A-5-149423 will result in an increase in the engine braking effect, with the negative torque transmitted from the drive wheels to the engine through the lock-up clutch. On the other hand, the vehicle driver releases the accelerator pedal when the driver desires to run the vehicle in the coasting mode or when the driver desires to bring the vehicle to a stop. If the slip control of the lock-up clutch and the fuel cut of the engine are simultaneously effected when the vehicle driver wishes to continue a coasting run of the vehicle, the driver may feel an engine braking effect while the driver has no intention to stop the vehicle. In this case, therefore, the simultaneous slip control of the lock-up clutch and fuel cut of the engine gives the driver a discomfort. The degree of deceleration of the vehicle as felt by the driver is relatively high if the fuel cut of the engine is initiated as soon as the accelerator pedal has been released to the engine idling position. In this respect, the fuel cut upon releasing of the accelerator pedal may deteriorate the driving comfort as felt by the vehicle driver.
As indicated above, the slip control output for controlling the lock-up clutch in the slip control mode during deceleration of the vehicle is determined according to a control equation which includes a feedback control value and a feed-forward control value. The feedback control value is used to control the lock-up clutch such that the actual slip speed of the clutch coincides with a predetermined target slip speed. The feed-forward control value is used to determine the target slip speed depending upon the input torque of the lock-up clutch or output torque of the engine. In the slip control apparatus disclosed in JP-A-5-141528, the control equation is formulated so that the amount of decrease of the engine speed coincides with a predetermined target value.
The slip control of the lock-up clutch is effected by controlling a difference between the hydraulic pressures in engaging and releasing oil chambers formed on the opposite sides of a piston of the lock-up clutch. This pressure difference of the two oil chambers is controlled on an assumption that the torque capacity of the lock-up clutch linearly changes with a change in the pressure difference. During deceleration of the vehicle, the engine speed is lower than the turbine impeller speed, and a negative torque is transmitted from the vehicle drive wheels to the engine through the lock-up clutch. In this negative torque condition, the operating conditions of the torque converter greatly differ from that in the normal positive torque condition. For instance, the pressure distribution within the torque converter and the amount of oil leakage between the pump and turbine impellers considerably differ in the negative and positive torque conditions. Further, since the transmission torque of the lock-up clutch is considerably smaller in the slip control mode of the lock-up clutch than in the fully engaging mode, a relatively small change in the pressure difference will cause a relatively large change in the slip speed of the lock-up clutch. Therefore, a slight change or deviation of the feed-forward control value tends to cause a temporary full engagement of the lock-up clutch (with the slip speed being zeroed). This temporary full engagement of the clutch caused by the feed-forward control is generally changed into the slip control state, in a certain length of time as a result of the feedback control of the slip speed of the clutch. If the vehicle speed is largely lowered by vehicle driver's brake application during such a temporary full or substantially full engagement of the lock-up clutch, the engine speed is accordingly lowered with a decrease in the turbine impeller speed. In this case, the engine may stall or have a knocking tendency. The possibility of the engine stall is particularly high when the fuel cut of the engine is effected together with the slip control of the lock-up clutch, because the fuel cut causes a sudden drop of the engine speed far below the fuel-cut speed, and the engine stalls before the fuel supply is resumed.